The present invention relates to telescopic props which can be extended or retracted hydraulically and which are particularly intended for use in mineral mines.
A known form of telescopic prop is disclosed in German Pat. No. 1,129,911. This known prop has three cylinder members disposed one within another in telescopic manner. The innermost and central or intermediate member has pistons facing working chambers in the outer and central members, respectively. These working chambers are interconnected through a non-return valve so that hydraulic pressure fluid can be admitted in succession to these chambers to extend the central member and then the inner member. In addition annular working chambers are provided between the outermost and central members and between the central and innermost members. The central and innermost members are provided with further piston surfaces facing these annular chambers to retract the inner member and then the central member. Bores are provided in the central member and in the piston of the innermost member so that during retraction the annular chamber between the central and innermost members receives fluid to retract the innermost member into the central member and thereafter communication between the annular chambers is established through the bores to allow fluid to enter the annular chamber between the outermost and central members to effect retraction of the central member into the outermost member.
A general object of the present invention is to provide an improved form of telescopic prop.
A further object is to provide a prop in which the central member is retracted within the outer member before the inner member is retracted within the central member.
Another object is to provide a prop in which the members will not inadvertently extend due to the formation of pressure heads in the working chambers during the retraction operation.